CVE-2026-33946 (mcp): MCP Ruby SDK - Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

ADVISORIES

GEM

mcp

SEVERITY

CVSS v3.x: 8.2 (High)

PATCHED VERSIONS

>= 0.9.2

DESCRIPTION

Summary

The Ruby SDK's streamable_http_transport.rb implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream and intercept all real-time data.

Details

Root Cause

The StreamableHTTPTransport implementation stores only one SSE stream object per session ID and lacks:

Session-to-user identity binding
Ownership validation when establishing SSE connections
Protection against multiple simultaneous connections to the same session

Impact

While the absence of user binding may not pose immediate risks if session IDs are not used to store sensitive data or state, the fundamental purpose of session IDs is to maintain stateful connections. If the SDK or its consumers utilize session IDs for sensitive operations without proper user binding controls, this creates a potential security vulnerability. For example: In the case of the Ruby SDK, the attacker was able to hijack the stream and receive all the tool responses belonging to the victim. The tool responses can be sensitive confidential data.

RubySec